It has been established that prostaglandin F2 alpha (PGF2 alpha elevates the concentration of circulating estrogens. Preliminary results suggest that x-irradiation of sheep ovaries substantially reduces the luteolytic effect of PGF2 alpha. From these data it would appear that an important element to the luteolytic action of PGF2 alpha is stimulatio of estrogen secretion. Further studies are planned in cattle to isolate the luteolytic action of PGF2 alpha by x-irradiation of ovaries and to determine the site of PGF2 alpha-induced estrogen secretion. Attempts will also be made to demonstrate a synergistic luteolytic action of estrogens and PGF2 alpha sheep, cattle and monkeys. Studies are also to be conducted on the effect of in vivo PGF2 alpha administratio on in vitro progesterone synthesis before and after PGF2 alpha- induced estrogen secretion. The formation of rapidly labelled RNA is to be studied under similar conditions.